Runners do it before a race. Swimmers do it before hopping in the pool. Your average gym rat does it before starting to pump iron. Most coaches, gym teachers, and personal trainers preach that stretching before exercise is an essential part of both avoiding injury and improving performance. But while it’s still popularly considered a basic tenet of health and wellness, scientific research into the value of stretching has cast doubt on its usefulness. For many athletic pursuits, studies suggest that stretching might actually be detrimental.

Even for those who follow fitness science closely, old habits die hard and many may be loath to give up stretching. “Because stretching decreases pain and makes you feel good, it is easy to extrapolate this to think it will prevent injury. But unfortunately, the reason it makes you feel good is one of the reasons it does not prevent injury,” states Dr. Ian Shrier, a sports medicine physician at McGill University in Montreal and author of many scientific studies on stretching.

So how did stretching become so ingrained in our collective athletic consciousness to begin with?

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The case for stretching as a means for injury prevention is grounded in the theory that a muscle that has been lengthened by stretching is more supple, decreasing stress to surrounding tendons, ligaments, and muscles and protecting them from the repeated stresses of activity. A study in the journal Sports Medicine advises that “improving flexibility through stretching is another important preparatory activity that has been advocated to improve physical performance. Maintaining good flexibility also aids in the prevention of injuries to the musculoskeletal system.”

Unfortunately, there is at best a tenuous connection between stretching and injury risk. “In general, stretching before exercise does not prevent injury,” Shrier says. “But we have not yet studied every possible context, so some people argue that it might still be beneficial for particular activities based on some theories.”

The question is not whether stretching actually results in a short-term increase in a person’s flexibility—it does—but whether that state confers protection against injury.

The most common mode of stretching, and the type that has come under fire, is referred to as static stretching. It involves lengthening a muscle and holding it in a mildly uncomfortable position, usually for somewhere between 15 and 60 seconds.

In activities with a high risk of overuse injuries, such as jogging or cycling, research has concluded that this kind of stretching really doesn’t lessen the chance of injury. Research presented in the British Journal of Sports Medicine concluded that there wasn’t any evidence to suggest that a stretching intervention was effective in preventing lower limb injuries in joggers. The same article even advised swimmers, who habitually stretch their shoulders and arms before exercise, to minimize stretching, particularly at the shoulders where excessive mobility can cause injury.

"We need only enough to move through a healthy range of motion—beyond that, we can strain muscles."

Some of the pushback against static stretching also comes from the concern that any roadblock to activity—like the time it takes to get a good pre-exercise stretch—makes some less likely to exercise. The time spent dutifully stretching before exercise could detract from another more potentially useful activity, like a good warm-up, strength training, or stability exercises.

While pre-exercise stretching has come into question, stretching outside of exercise does seem to have some value, especially as aging decreases flexibility. Flexibility generally describes the range of motion commonly present in a joint. Although stretching can enhance flexibility, it’s an attribute that decreases with age, and varies by gender and ethnic group.

But being more flexible is not always better. “There’s certainly such a thing as too much flexibility,” says Sage Rountree, author of The Runner’s Guide to Yoga. “We need only enough to move through a healthy range of motion—beyond that, we can strain muscles and tendons and even destabilize ligaments and compromise joint health.”

Science has long known that the most flexible individuals are more likely to suffer injuries than an individual of average flexibility—one study of U.S. Army recruits found that those with the most and least flexibility more than doubled their risk of injury.

When it comes to static stretching, research has now repeatedly shown that stretching before exercise is counterproductive and results in a temporary loss of muscle strength. These losses can be as much as 5 percent and are magnified in those that hold a stretch for longer than 45 seconds.

Static stretching may slightly reduce the risk of muscular strains in sports characterized by explosive movement like football, but studies that found this decrease in muscle strain also noticed an increase in ligament sprains and other injuries after stretching.

In addition to the risk of injury, some studies suggest that stretching hurts performance as well—it’s associated with an acute loss in muscular strength. A comprehensive review by Canadian researchers presented in the European Journal of Applied Physiology concluded: “Based on the majority of the literature, it would seem logical to recommend that prolonged static stretching not be performed prior to a high level or competitive athletic or training performance.”

While stretching’s effect on performance seems to be most significant for those that sprint, jump or burst off of starting blocks, negative effects have been found for runners and cyclists as well. It seems that stretched muscles are less efficient, at least for a time, lowering a runner or cyclists’ fuel economy. After a pre-run or pre-cycle stretch, endurance athletes have to work harder to go at the same pace as someone who hasn’t stretched. Notably, in one such experiment, after first either stretching or quietly resting, runners were asked to complete a one-mile run as fast as possible. After stretching the runners ran an average of 13 seconds slower in the mile test.

Research has repeatedly shown that stretching before exercise is counterproductive and results in a temporary loss of muscle strength.

A plausible explanation for this phenomenon is that stretching may make the muscle more compliant, and could impair its neurologic function. In layman’s terms, like an old rubber band that’s lost its elasticity, a muscle can’t contract as forcefully after it has been stretched.

Of course, there are sports for which muscle flexibility is more important than strength. Stretching to develop flexibility is most useful for those individuals whose sport demands a high degree of motion, like ballet, gymnastics or figure skating, during which participants need to stretch to achieve the required motion for their activity. Shrier agrees, saying, “If you can kick the opponent in the head in a martial arts match with stretching but not without stretching, your performance is increased even though the power behind the kick is less.”

While many recreational fitness enthusiasts might not notice if a run took 45 seconds longer than normal, most sports medicine experts recommend a different pre-exercise activity to replace stretching. For both the weekend warrior and the professional athlete, that activity would be a warm-up. Warming up with a light jog or gentle bike ride, for example, has been shown to enhance subsequent performance.

“I strongly recommend people warm up as they start activity and increase the intensity gradually,” Shrier says. “In general, if they stretch before activity, I suggest they stop and replace it with more warm-up because most people simply do not warm up enough.”

For those who are married to their pre-workout stretch, dynamic stretching, which incorporates more the characteristics of warming-up, uses constant, fluid motion, which doesn’t trigger the same performance deficits as static stretching, in which the end positions are held and sometimes taken to the point of discomfort.

“Before a workout, repetitive movement takes the body through a wide range of motion and primes the muscles for the activity to come,” Rountree says, suggesting some dynamic stretches. “Pre-run, for example, we use lunges to simulate the running stride and prep the muscles; before a tennis match, pulsing in and out of a squat and circling the arms will get you ready to play.”